1. Technical Field
The invention concerns telecommunications, and more particularly, the generation of charge pulses for transmission to respective telephone network subscribers.
2. Description of the Prior Art
A circuit for generating charge pulses for indicating telephone charges during analog telecommunications is known from German patent document DE 35 16 007 C2. A telephone exchange controls a corresponding charge indicator by means of special charge signals via telephone lines. To adapt the charge signal level to the user line and to its specific total impedance, a signal which is an image of the total call signal and the charge signal is obtained from a point in the circuit. The thus obtained total signal is rectified and supplied to a comparator, which compares it with a reference signal. If the amplitude of the rectified signal is greater than the amplitude of the reference signal, a signal is generated whose level is kept constant with the use of a memory element.
With digital telecommunications, such as the digital telecommunications disclosed in German patent document DE 36 36 563 A1, the telephone exchange is unable to transmit 16 kHz charge pulses. Instead, a charge pulse must be generated after a corresponding signal is received directly by the subscriber or by an interface unit (SU), to which the subscriber's telephone is connected through an electrical service line. The subscriber's telephone must receive a sufficiently high voltage, for example 140 mV, from the interface unit, even with a maximum length of the service line between the interface unit and the subscriber's telephone. To that end the interface unit must generate a charge pulse with a level of 4.2 Volt for example. With a charge pulse of this level, the maximum length of the service line is then for example 4.5 km with a cable containing 0.4 mm diameter conductors.
On the one hand, all elements of the interface unit including the power source or power sources must be designed for maximum voltage. On the other, the extreme case of no-load running must be taken into consideration if a very high-impedance telephone with a very short service line is directly connected at 16 kHz to the interface unit. Problems with respect to the interface unit do not take place when such a high voltage pulse is transmitted to a telephone, provided that a maximum length service line interconnects the telephone to the interface unit. The impedance of such maximum length service lines represents a sufficient "load" for the interface unit. However, in shorter service lines, the control range of the power source of the respective subscriber can be exceeded by the alternating-voltage amplitude provided by the service line from the interface unit. This happens for example with the above-mentioned no-load running when the sum of the supply voltage of the telephone and its superimposed alternating-voltage for the charge pulse reaches or even exceeds the controlled bias voltage of the power source.